A public cloud computing system (“a cloud”) provides shared computing resources for use by customers. The computing resources of a cloud are hardware and software resources. The hardware resources include components of servers such as cores of central processing units (CPUs), graphics processing units (GPUs), main memory, secondary storage, and so on. The software resources include operating systems, database systems, accounting applications, and so on. A typical cloud may have several data centers at various locations throughout the world. Each data center may host tens of thousands of servers.
To use a cloud, a customer typically purchases a subscription to use the services of the cloud. When purchasing a subscription, a customer may provide billing information and be provided an account that is accessible using logon information such as a user name and password. To encourage users to become customers, a cloud provider may offer various incentives that allow the users to subscribe and use the cloud for a limited time. Once logged on, a customer can then use the servers of the cloud to execute computer programs such as for hosting websites, performing accounting functions, performing data analyses, and so on. A cloud may use various billing models such as model based on amount of core usage, memory usage, and other resource usage.
Clouds, like other computer systems, are susceptible to cyber-attacks. These cyber-attacks may include viruses, worms, denial-of-service attacks, and so on. Clouds are also susceptible to fraudulent use of resources resulting from exploitation of a vulnerability in the subscription process of the cloud. For example, a cloud may offer free 30-day subscriptions to new customers. When subscribing, the user may be provided with a Completely Automated Public Turing test to tell Computer and Humans Apart (“CAPTCHA”) test. If the user discovers a vulnerability in the subscription process that allows the user to bypass or always pass the CAPTCHA test, that user may be able to develop a computer program to create hundreds and thousands of new, but unauthorized, subscriptions. As another example, a user may have access to valid, but stolen, credit card numbers. Such a user can create hundreds of subscriptions using the stolen credit card numbers before the theft is identified. Such unauthorized users can then use computer resources at such a high rate that there are insufficient resources left to service the needs of authorized users. To help lessen the impact of such unauthorized uses, a cloud may be designed with a certain amount of capacity that is in excess of what is needed to support authorized users. The price of purchasing and maintaining such excess capacity can be high.